Size tunable biosynthesis and luminescence quenching of nanostructured hematite (α-Fe2O3) for catalytic degradation of organic pollutants

In the present study, an environmentally benign protocol has been proposed for surfactant-free and template-free aqueous biosynthesis of hematite nanoparticles without complicated heat treatment procedures. Nanostructured hematite of different sizes has been synthesized, exploiting the capping potential of polyphenolic tannin rich leaf broth of Psidium guajava plant, by optimizing the precursor concentration and quantity of broth. The synthesized nanostructures are characterized by XRD, SEM, TEM, HRTEM, EDX and FTIR techniques to confirm their phase purity, size distribution and chemical environment. In order to determine the proper annealing temperature, thermal analysis of the as synthesized product has been carried out. The absorption and reflection characteristics of the nanoparticles in the UV-vis-IR region confirm the existence of energy band gap of 2.1 eV in the samples. Magnetic characteristics of the samples have been investigated using Vibrating Sample Magnetometer to show weak ferromagnetism at room temperature, tendency of a superparamagnetic behavior at lower temperatures and size dependent coercivity and saturation magnetization. The relatively small difference between the irreversibility temperature and blocking temperature observed in FC and ZFC curves supports the fact that the nanoparticles are distributed in a small size range, which is in agreement with SEM and TEM result. The synthesized nanoparticles are found to show luminescent emission in the visible region at 575 nm on excitation by a wavelength of 384 nm. The capping of acidic tannin molecules could effectively quench the luminescence of the synthesized particles, thereby enhancing their catalytic efficiency. The biosynthesized nanoparticles could effectively catalyse degradation of the organic pollutants methyl red, methyl orange, eosin yellowish and methylene blue which recommends the possible application in the treatment of industrial waste water.